In recent years much effort has been devoted to the biological production of renewable fuels such as ethanol. However, ethanol is not an ideal fuel, suffering from problems such as high hygroscopicity, high vapor pressure and low energy density. These qualities make ethanol incompatible with the current facilities used in the storage, distribution and use of liquid transportation fuels.
Higher-chain (C≧4) alcohols (e.g. n-butanol), fatty acid methyl esters (FAMEs) and hydrocarbons (alkanes and alkenes) offer several advantages compared to ethanol, including reduced hygroscopicity, reduced volatility, and higher energy density. These qualities make n-butanol and other higher alcohols more compatible with our current infrastructure for storage, distribution and usage.
The aforementioned long-chain fuels and chemicals are generated from short-chain metabolic intermediates through pathways that require carbon-chain elongation. However, biological efforts to date have been less than satisfactory, particularly where non-native genes are introduced to drive synthesic of longer chain molecules.
Therefore, what is needed in the art are better biological methods of making higher-chain (C≧4) fuels (e.g. alcohols, fatty acid methyl esters, FAMEs, and hydrocarbons) that are more efficient and cost effective than are currently available. The ideal method would also allow the production of chemicals, such as carboxylic acids and alcohols, that can be used as feedstocks in other industries.